The mechanisms by which chemical agents inhibit the development of tumors in animals are complex and incompletely understood. Thromboxanes, prostacyclin, and prostaglandins exhibit effects which may be important in regulating cell transformation and proliferation. We and others have found that thromboxanes stimulate DNA synthesis, lower cAMP levels, and stimulate proteinase release whereas prostacyclin inhibits DNA synthesis, elevates cAMP levels, and induces differentiation. Thus, modulation of the thromboxane-prostacyclin ratio may be an important mechanism for the control of cell proliferation. Thromboxanes and prostacyclin are formed from a hydroxy endoperoxide, PGH2, which is derived from arachidonic acid. During the oxygenation of arachidonic acid, oxidizing radicals are produced as a result of the interaction of a hydroperoxy endoperoxide, PGG2, with the hydroperoxidase component of prostaglandin endoperoxide synthetase. These radicals inactivate prostacyclin synthetase thereby reducing the capability of cells to produce prostacyclin. Compounds which serve as cofactors for the reduction of hydroperoxides inhibit the formation of oxidizing radicals by lowering the steady state level of PGG2 and maximize prostacyclin biosynthetic capability. Therefore, this may represent a mechanism for the alteration of the thromboxane-prostacyclin ratio by chemopreventive agents in addition to the inhibition of arachidonic acid oxygenation. We propose to determine which chemopreventive agents are reducing cofactors for the hydroperoxidase component of prostaglandin endoperoxide synthetase, stimulate prostacyclin synthetase, and prevent oxidizing radical formation in vitro. We will also determine which chemopreventive agents modulate the thromboxane-prostacyclin ratio in mouse skin in vivo and in Syrian hamster embryo fibroblasts and C3H cells in culture and determine how this correlates to proteinase release and cell transformation. These experiments will determine whether modulation of the thromboxane-prostacyclin ratio is an important mechanism of action of chemopreventive agents.